1. Field of the Invention
The present invention relates to an alloy sheet for making a shadow mask used for a color cathode ray tube and a method for manufacturing thereof, in particular to an alloy sheet containing Fe, Ni and Cr having high press-formability and a method for manufacturing thereof.
2. Description of the Related Arts
The recent trend of up-grading of color television towards high definition TV has employed an Fe-Ni alloy containing 34 to 38wt. % Ni as the alloy for making a shadow mask to suppress color-phase shift, Compared with low carbon steel which has long been used as a shadow mask material, conventional Fe-Ni alloy has considerably lower thermal expansion coefficient. Accordingly, a shadow mask made of conventional Fe-Ni alloy raises no problem of color-phase shift coming from the thermal expansion of a shadow mask even when an electron beam heats the shadow mask.
The common practice of making the alloy sheet for shadow mask includes the following steps. An alloy ingot is prepared by continuous casting process or ingot-making process. The alloy ingot is subjected to slabbing, hot-rolling, cold-rolling, and annealing to produce an alloy sheet.
The alloy sheet for making a shadow mask is then processed usually in the following steps to form shadow mask. (1) The alloy sheet is photo-etched to form passage-holes for the electron beam on the alloy sheet for making a shadow mask. The alloy sheet for making a shadow mask perforated by etching is hereinafter referred to as a "flat mask". (2) The flat mask is subjected to annealing. (3) The annealed flat mask is pressed into a curved shape of a cathode ray tube. (4) The press-formed flat mask is assembled to a shadow mask which is then subjected to a blackening treatment.
However, the above mentioned conventional Fe-Ni alloy has higher strength and plane anisotropy as mechanical properties than conventional low carbon steel. Accordingly, an alloy sheet for a shadow mask prepared by the process including first cold-rolling, recrystallization annealing and finish cold-rolling, which is annealed before press-forming after being perforated by etching, develops problems such as poor shape-fixability, cracking on the alloy sheet and blurred periphery of the pierced hole during press-forming, which is a significant disadvantage of manufacturing a cathode ray tube.
In addition, the above described Fe-Ni alloy gathers rust easily, which reduces production yield druing the manufacturing process of shadow a mask.
The prior art was proposed in JP-A-3-267320 (the term JP-A- referred to herein signifies an unexamined Japanese patent publication), where a method to decrease the strength of the conventional Fe-Ni alloy and solve the above problems is provided. Hereafter, said art is referred to as prior art 1. The prior art 1 employs first cold-rolling, recrystallization annealing, finish cold-rolling and softening annealing. The finish cold-rolling is performed at a reduction ratio of 5 to 20%, and the temperature of the softening annealing is less than 800.degree. C. The prior art 1 produces a sheet having sufficiently low strength to give good press-formability with the 0.2% proof stress of 9.5 kgf/mm.sup.2 (10 kgf/mm.sup.2 or less) at 200.degree. C.
The prior art 2 was proposed in JP-B-64-52024, where a method to reduce a plane anisotropy as a mechanical property is provided. To produce a plate for a shadow mask having the less plane anisotropy of elastic coefficient, the prior art 2 employs a process including cold-rolling followed by recrystallization annealing repeated twice or more, and cold-rolling to increase hardness. In that process, finish cold-rolling before finish recrystallization annealing is performed at the reduction ratio of 40 to 80%. The plate for a shadow mask has an excellent uniform formability during press-forming resulting in a small deformation of etched-hole and free from the irregular gloss and stringer detect when the plate is etched, annealed and press-formed.
However, the prior art 1 does .not satisfy the quality required to perform a favorable warm press-forming, though the strength is lowered to the level suitable for press-forming under the above described annealing condition. The alloy sheet for a shadow mask prepared by the prior art was found to gall a die and to generate cracks at the edge of shadow masks. Furthermore, the above described alloy sheet for making a shadow mask often develops a quality problem of blurred periphery of pierced holes after press-forming, as the plane anisotropy of the alloy is large.
In the prior art 2, the plane anisotropy of elastic coefficient of the plate is small, and the blurred periphery of pierced holes according to the deformation of the holes during the press-forming is not observed. Still, the prior art 2 induces cracks at the edge of shadow mask during press-forming and does not improve the corrosion resistance of Fe-Ni alloy.
Present color televisions require severer quality specifications on color-phase shift because the color picture tubes directed to brighter and more flat faces than ever. The cathode ray tubes using the shadow masks prepared by the prior arts mentioned above give partial color-phase shift under electron beam irradiation.